Buggy code:
public class UnitChecker { private static UnitChecker unitchecker ; public static Unit null_unit = new Unit ( 0 , "null" , 1 ) ; public static Unit dimensionless_unit = new Unit ( 0 , "dimensionless" , 1 ) ; public static Collection < Unit > null_collection = new ArrayList < Unit > ( ) ; public static Collection < Unit > dimensionless_collection = new ArrayList < Unit > ( ) ; public static ArrayList < UnitEquivalence > base_equivalences = new ArrayList < UnitEquivalence > ( ) ; public static ArrayList < UnitEquivalence > reciprocal_equivalences = new ArrayList < UnitEquivalence > ( ) ; public static Collection < UnitEquivalence > equivalences = new ArrayList < UnitEquivalence > ( ) ; public static UnitChecker getUnitChecker ( ) { } private static void populate_equivalences ( ) { } private static boolean add_to_base ( UnitEquivalence new_ue ) { } private static void generate_transitive_equivalences ( ) { } private static ArrayList < Unit > simplify ( Collection < Unit > array ) { } private static boolean contains_equivalence ( ArrayList < UnitEquivalence > equivs , UnitEquivalence ue ) { } private static boolean matches ( Collection < Unit > first , Collection < Unit > second ) { } private static boolean matches ( Unit first , Collection < Unit > second ) { } private ArrayList < Unit > expandUnits ( Collection < Unit > units ) { } public float CheckUnitCompatability ( Collection < Unit > first , Collection < Unit > second ) { if ( ( first == null ) || ( second == null ) ) return 0 ; ArrayList < Unit > first_array = new ArrayList < Unit > ( ) ; for ( Unit u : first ) { first_array . add ( u ) ; if ( ( u . getName ( ) . equals ( "null" ) ) || ( u . getName ( ) . equals ( "dimensionless" ) ) ) return 1 ; } ArrayList < Unit > second_array = new ArrayList < Unit > ( ) ; for ( Unit u : second ) { second_array . add ( u ) ; if ( ( u . getName ( ) . equals ( "null" ) ) || ( u . getName ( ) . equals ( "dimensionless" ) ) ) return 1 ; } float scale = 1 ; if ( ( DisplayOptions . getOptions ( ) . ATTEMPT_TYPE_CONVERSION ) && ( ( base_form ( first_array ) ) != null ) ) { UnitBaseForm ubf = base_form ( first_array ) ; if ( ubf . getBaseform ( ) . contains ( UnitChecker . dimensionless_unit ) ) return 1 ; first_array = ubf . getBaseform ( ) ; if ( scale == 1 ) scale = ubf . getScalefactor ( ) ; else scale *= ubf . getScalefactor ( ) ; } if ( ( DisplayOptions . getOptions ( ) . ATTEMPT_TYPE_CONVERSION ) && ( ( base_form ( second_array ) ) != null ) ) { UnitBaseForm ubf = base_form ( second_array ) ; if ( ubf . getBaseform ( ) . contains ( UnitChecker . dimensionless_unit ) ) return 1 ; second_array = ubf . getBaseform ( ) ; scale //= ubf . getScalefactor ( ) ; } ArrayList < Unit > leftover_array = new ArrayList < Unit > ( ) ; for ( int i = 0 ; i < ( first_array . size ( ) ) ; i ++ ) { Unit fu = first_array . get ( i ) ; boolean found = false ; found = UnitChecker . check_for_unit ( second_array , fu ) ; if ( ! found ) { leftover_array . add ( fu ) ; } } if ( ( DisplayOptions . getOptions ( ) . ATTEMPT_TYPE_CONVERSION ) && ( ( leftover_array . size ( ) ) > 0 ) ) { leftover_array = expandUnits ( leftover_array ) ; if ( ( second_array . size ( ) ) == 0 ) return 0 ; scale = check_equivalences ( leftover_array , expandUnits ( second_array ) ) ; return scale ; } <START_BUG> return ( second_array . size ( ) ) == 0 ? scale : 0 ; <END_BUG> } private float check_equivalences ( ArrayList < Unit > leftover_array , ArrayList < Unit > second_array ) { } private Collection < Unit > inverse ( ArrayList < Unit > units ) { } private boolean contains_units ( Collection < Unit > container , Collection < Unit > contained ) { } private UnitBaseForm base_form ( Collection < Unit > units ) { } private boolean isBaseUnit ( Unit u ) { } private boolean allBaseUnits ( Collection < Unit > units ) { } private static boolean check_for_unit ( ArrayList < Unit > array , Unit u ) { } public Collection < Unit > multiply_units ( Collection < Unit > first , Collection < Unit > second ) { } public Collection < Unit > divide_units ( Collection < Unit > first , Collection < Unit > second ) { } public Collection < Unit > add_units ( Collection < Unit > first , Collection < Unit > second ) { } private static ArrayList < Unit > clone_unit_list ( Collection < Unit > first ) { } private Collection < Unit > combine_units ( ArrayList < Unit > first_array , ArrayList < Unit > second_array ) { } public boolean contains_null ( Collection < Unit > units ) { } public Collection < Unit > power_units ( Collection < Unit > units , float value ) { } public boolean isDimensionless ( Collection < Unit > units ) { } public Collection < String > getEquivalences ( ) { } public void remove_base_equivalence ( int remove ) { } public boolean add_equivalence ( Collection < Unit > first , float scale_factor , Collection < Unit > second ) { } public String getOutputEquivalence ( ) { } }

Fix:
return ( ( second_array . size ( ) ) == 0 ) && ( ( leftover_array . size ( ) ) == 0 ) ? scale : 0 ;